Frequency up/down converters (referred to below as up/down converters) are known as a technology for frequency conversion. FIG. 1 shows a diagram of a conventional up converter and FIG. 2 shows a diagram of a conventional down converter.
First, a conventional up converter will be described while referring to FIG. 1. First local oscillator LO1124 of the up converter in FIG. 1 is typically a low phase-noise signal source of a fixed frequency and second local oscillator LO2126 is a low phase-noise signal source of a variable frequency. In FIG. 1, signals given to input terminal 110 are passed through BPF (band pass filter) 112 and mixed with signals from LO1124 (of a predetermined fixed frequency) by mixer 114 and the frequency component of the difference between the two signals is removed, that is, the frequency component of the sum of the two signals, is produced by BPF 116. There is further mixing with signals output at a predetermined frequency from LO2126 by mixer 118 and the frequency component of the sum of the two signals is removed by LPF (low pass filter) 120 and output to output terminal 122.
In this case, assuming that signal f110 given to input terminal 110 isf110=cos(w—if*t)  (1) as a signal without phase noise, signal f124 output from LO1 isf124=cos(w2*t+θpn2(t))  (2) with phase noise as θpn2(t), and signal f126 from LO2 isf126=cos(w3*t+θpn3(t))  (3) with phase noise as θpn3(t),first intermediate frequency signal IF1 at the output of mixer 114 is the product of both signals f110 and f124. To summarize,                                                         IF1              =                            ⁢                                                cos                  ⁡                                      (                                          w_if                      *                      t                                        )                                                  *                                  cos                  ⁡                                      (                                                                  w2                        *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn2                          ⁡                                                      (                            t                            )                                                                                                                )                                                                                                                          =                            ⁢                              cos                (                                                      cos                    ⁡                                          (                                                                                                    (                                                          w_if                              +                              w2                                                        )                                                    *                          t                                                +                                                  θ                          ⁢                                                                                                           ⁢                                                      pn2                            ⁡                                                          (                              t                              )                                                                                                                          )                                                        +                                                                                                                                        ⁢                                  cos                  ⁡                                      (                                                                                            (                                                      w_if                            -                            w2                                                    )                                                *                        t                                            -                                              θ                        ⁢                                                                                                   ⁢                                                  pn2                          ⁡                                                      (                            t                            )                                                                                                                )                                                  )                            /              2                                                          (        4        )            Furthermore, in this specification, the formulas representing signals that have passed through mixers or filters focus on the relationship with frequency for what needs explanation, but persons skilled in the art can alter the formulas as needed for amplitude, and therefore, the details are not given special mention. Therefore, the phrase “to summarize” has been used when the formulas are introduced.
Next, the lower frequency components are removed by BPF 116, and therefore, signal f116 at the output of BPF 116 becomesf116=cos((w—if+w2)*t+θpn2(t))  (5) 
When frequency is converted by mixer 118, second intermediate frequency signal IF2 at the output of mixer 118 becomes                                                         IF2              =                            ⁢                                                cos                  ⁡                                      (                                                                  w3                        *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn3                          ⁡                                                      (                            t                            )                                                                                                                )                                                  *                                  cos                  ⁡                                      (                                                                                            (                                                      w_if                            +                            w2                                                    )                                                *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn2                          ⁡                                                      (                            t                            )                                                                                                                )                                                                                                                          =                            ⁢                              (                                                      cos                    ⁡                                          (                                                                                                    (                                                          w3                              +                              w_if                              +                              w2                                                        )                                                    *                          t                                                +                                                  θ                          ⁢                                                                                                           ⁢                                                      pn3                            ⁡                                                          (                              t                              )                                                                                                      +                                                  θ                          ⁢                                                                                                           ⁢                                                      pn2                            ⁡                                                          (                              t                              )                                                                                                                          )                                                        +                                                                                                                                        ⁢                                  cos                  ⁡                                      (                                                                                            (                                                      w3                            -                            w_if                            -                            w2                                                    )                                                *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn3                          ⁡                                                      (                            t                            )                                                                                              -                                              θ                        ⁢                                                                                                   ⁢                                                  pn2                          ⁡                                                      (                            t                            )                                                                                                                )                                                  )                            /              2                                                          (        6        )            
The higher frequency components are removed by LPF 120 here, and therefore, signal f120 at the output of LPF 120 becomesf120=cos((w3−w—if−w2)*t+θpn3(t)−θpn2(t))  (7) 
In formula (7), since θpn3(t) and θpn2(t) are non-correlated noise from different signal sources, these phase noises are added to the signal f120 to output terminal 122 and given to output. The results relating to phase noise here would not be different, if the component of lower frequency from first intermediate frequency IF1 was used.
Next, a conventional down converter will be described while referring to FIG. 2. First local oscillator LO1226 of the down converter in FIG. 2 is a low phase-noise signal source of variable frequency and second local oscillator LO2224 is a low phase-noise signal source of fixed frequency. The signal given to input terminal 222 passes through LPF 220 in FIG. 2 in order to remove images of frequency components and is mixed with a signal of a predetermined frequency from first local oscillator LO1226 by mixer 218. The frequency component of the sum of both signals is removed by BPF 216, and further mixing with signals (of a predetermined fixed frequency) from second local oscillator LO2224 is performed by mixer 214. The frequency component of the sum of the two signals is removed by BPF 212 and, then the result is output to output terminal 210.
In this case, assuming that input signal f222 of input terminal 222 isf222=cos(w—in*t)  (8) as a signal without phase noise, signal f226 from first local oscillator LO1 isf226=cos(w1*t+θpn1(t))  (9) with phase noise as θpn1(t), and signal f224 from second local oscillator LO2224 isf224=cos(w2*t+θpn2(t))  (10) with phase noise as θpn2(t), first intermediate frequency signal IF1 at the output of mixer 218 becomes the product of input signal f222 and signal f226 from LO1 and                                                         IF1              =                            ⁢                                                cos                  ⁡                                      (                                                                  w1                        *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn1                          ⁡                                                      (                            t                            )                                                                                                                )                                                  *                                  cos                  ⁡                                      (                                          w_in                      *                      t                                        )                                                                                                                          =                            ⁢                              (                                                      cos                    ⁡                                          (                                                                                                    (                                                          w1                              +                              w_in                                                        )                                                    *                          t                                                +                                                  θ                          ⁢                                                                                                           ⁢                                                      pn1                            ⁡                                                          (                              t                              )                                                                                                                          )                                                        +                                                                                                                                        ⁢                                  cos                  ⁡                                      (                                                                                            (                                                      w1                            -                            w_in                                                    )                                                *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn1                          ⁡                                                      (                            t                            )                                                                                                                )                                                  )                            /              2                                                          (        11        )            
Here, the higher component is removed by BPF 216 and therefore, signal f216 at the output of BPF216 becomesf216=cos((w1−w—in)*t+θpn1(t))  (12) 
Next, when frequency is converted by mixer 214, second intermediate frequency signal IF2 at the output of mixer 214 becomes                                                         IF2              =                            ⁢                                                cos                  ⁡                                      (                                                                                            (                                                      w1                            -                            w_in                                                    )                                                *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn1                          ⁡                                                      (                            t                            )                                                                                                                )                                                  *                                  cos                  ⁡                                      (                                                                  w2                        *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn2                          ⁡                                                      (                            t                            )                                                                                                                )                                                                                                                          =                            ⁢                              (                                                      cos                    ⁡                                          (                                                                                                    (                                                          w1                              -                              w_in                              +                              w2                                                        )                                                    *                          t                                                +                                                  θ                          ⁢                                                                                                           ⁢                                                      pn1                            ⁡                                                          (                              t                              )                                                                                                      +                                                  θ                          ⁢                                                                                                           ⁢                                                      pn2                            ⁡                                                          (                              t                              )                                                                                                                          )                                                        +                                                                                                                                        ⁢                                  cos                  ⁡                                      (                                                                                            (                                                      w1                            -                            w_in                            -                            w2                                                    )                                                *                        t                                            +                                              θ                        ⁢                                                                                                   ⁢                                                  pn1                          ⁡                                                      (                            t                            )                                                                                              -                                              θ                        ⁢                                                                                                   ⁢                                                  pn2                          ⁡                                                      (                            t                            )                                                                                                                )                                                  )                            /              2                                                          (        13        )            and when the higher component is dropped by BPF 212, signal f212 at the output of BPF 212 becomesf212=cos((w1−w—in−w2)*t+θpn1(t)−ƒpn2(t)) (14) 
In formula (14), since θpn3(t) and θpn2(t) are non-correlated noise from different signal sources, these phase noises are added to output signals and are given to output.
Accordingly, there is a disadvantage with conventional up/down converters in that phase noise from internal signal sources is obtained in the output and it becomes necessary to use a high-performance internal signal source with low phase-noise in order to keep phase noise to lower.
In particular, signal sources 124 and 224 of fixed frequency in conventional up/down converters must have as high an output frequency as signal sources 126 and 226 of variable frequency. In general, as higher the output frequency becomes in a signal source, the phase noise level also increases. Therefore, a signal source with low phase-noise is needed, even when used as the signal source of fixed frequency, and as a result, the up/down converter becomes expensive.
Therefore, when this type of conventional converter is used in systems such as IC testers, etc., the cost of developing a dedicated fixed frequency signal source increases and becomes expensive. Moreover, output frequency cannot be varied, and therefore, this signal source cannot be flexibly used with other modules. Consequently, the problems with these conventional converters are two-fold.
Therefore, there was a need for a high-frequency up/down converter with which phase noise is reduced even if a low-cost ordinary signal source is used.